1. Field of the Invention
This invention relates to an examined body interior information observing apparatus high in sensitivity and favorable in S/N by using photo-pulses for obtaining information of an examined body interior.
2. Description of Related Art
Recently, the importance of vein contrasting has increased along with the increase of circulatory system and brain vein system diseases and the prevalence of the utilization of images in the diagnosis. However, although the vein contrasting has become comparatively easy with the progress of digital radiography, in case it is applied to a human body, the danger and the pain of the examinee should not be neglected.
The information within an examined body such as a living body has been non-invasively measured mostly by X-rays without contact. However, regarding the use of X-rays, there are known problems regarding the influence of radioactive rays and the difficulty of imaging the living body functions. There are also problems in that the apparatus of the NMR-CT method is large and costly and the perspective obtained by ultrasonic waves is low in spatial resolution.
Now, it is known that a hemoglobin (Hb) in blood shows a peculiar spectral variation in response to the oxygenizing degree against the light in the near infrared ray region. By utilizing this feature, as shown, for example, in the article "Living Body Measurement by Using Light" mentioned in the magazine "O plus E", May 1987 to March 1988, the research relating to such non-invasive measurement of living body interior information, such as the blood oxygen saturated degree measurement, are being actively made. It is also shown in the article "Basic Investigation Relating to Visualizing Veins within Living Bodies by Near Infrared Rays" in the Technical Research Report of Electronic Information Communication Society that the hemoglobin (Hb) in blood is so high in the degree of extinction in the infrared ray region than living body tissue as to be able to detect the vein in the tissue as an image by using light.
Aside from the above-mentioned method of observing the interior of a living body from outside the body by using a transmitted light, as shown in the publication of Japanese Patent Application Laid Open No. 85417/1988 and in the article "Femtosecond Optical Ranging in Biological System" in the Optics Letters, Vol. 11, No. 3, 1986, pp. 150 to 152, the interior can be measured also by reflected light. Therein, a light of a pulse width so short as to be of a half value width of several hundred femto- to several pico-second is radiated from a light source to an examined body and the variation over time of the intensity of the reflected light is measured to observe the interior of a living body. In other words, as the lapse of time until the light reflected from a structure at a certain depth from the surface of the living body returns corresponds to the depth, the state of the blood distribution and structure of the living body interior tissue can be measured.
However, it is disclosed in the article "Application of the 1-D diffusion approximation to the optics of tissues and tissue phantoms" in the Appl. Opt., Vol. 28, 1989, pp 2311 to 2317 that near infrared light rays are higher in transmittivity than visible light rays, but the intensity of the transmitted light will attenuate by about 1 to 2 figures even in case the transmitted light passes through a thickness of 1 cm of a muscular tissue, for example, within a living body. In other words, in the above-described method of detecting the reflected light, the deeper in the living body, the lower the intensity of the reflected light due to the absorption and dispersion by the living body tissue and the detected reflected light intensity will no longer correspond to the actual light intensity from the deep part. Also, light intensity from the deeper part will be so low that, with an ordinary detector, the sensitivity will be low and the S/N will be severely attenuated. In such case, it will be considered easy to increase the sensitivity of the detector to detect a feeble signal from the deeper area. However, the strong reflected light from the part near the surface will be simultaneously detected, therefore the signals will be saturated over the allowable detectable range of the detector and, in the worst case, the detector wills seize.